When Rue tricks the NA meeting representative into signing her attendance for the dates that she missed, there are more signatures than when she presents the sheet to her mother in the next scene.
Kat's meeting with the principal would not have taken place without her parents present.
It makes no sense for the teacher to grill Rue about a personal experience she had during the recent summer break, and put her on the spot like that. It would not even make sense to grill an adolescent that way who has not had a drug OD and near-death experience; many students that age are notoriously sensitive about personal matters and there is no discernible learning goal to the whole exercise. There might be an educational goal but then the normal thing for the teacher to do is obviously to wait for volunteers, not order a student to tell a personal story.
But in this case, the teacher would have been made aware of Rue's issues as a matter of course, for very obvious reasons. Teachers need that kind of information to be able to do their job, and they are bound to keep it confidential by law. The teacher could therefore very easily lose her job over this, and indeed her entire teaching career, especially if Rue's mother decided to sue her, as many parents would.
It makes no sense for Kat to be able to browbeat the principal like that. You do not get to be high school principal by being a complete pushover to teenagers.